Fusion researchers demonstrate self-organizing plasma

When University of Wisconsin-MadisonEngineering Physics Professor Ray Fonck and his students built a tokamak-style fusion experiment nearly 12 years ago, they hoped the prototype reactor would demonstrate the potential of a very-low-aspect-ratio design that may allow researchers to develop smaller fusion systems in the future.

Now, the researchers have demonstrated a technique that enables them to start Pegasus and create a stable plasma without using a solenoid magnet in the center.

The method, which incorporates a plasma torch developed by UW-Madison physics researchers, addresses limits on magnetic field capacity in low-aspect-ratio tokamaks and could scale up to some of the world’s largest tokamak experiments.

The UW-Madison researchers published details of their advance in the June 5 issue of Physical Review Letters. They refer to their method as “lighting the match” to create a hot plasma.

A tokamak device is shaped somewhat like a chunky ring-style doughnut with a hole in the center. Powerful magnets encircle the device both around the long, or toroidal, direction, as well as vertically to help confine and stabilize the plasma. The solenoid magnet runs through the center hole and — through standard magnetic induction by continuously increasing the magnetic field strength — drives a plasma current to heat and confine the plasma. “The problem is, that doesn’t work for the future, because you can’t just keep increasing the magnetic field,” says Fonck. “Eventually, you reach the physical limits of the magnets and the power supplies. So, there’s always been a need to find a way to start these tokamak plasmas without inductive current from a central solenoid, and to sustain them without inductively driven current.”

Fonck and his students ran up against that very wall several years ago. “We found the plasmas we could get in Pegasus were limited in how high they could go in pressure — not because of the stability limits we wanted to study, but because of our limited ability to drive the current,” he says.

Although Pegasus boasts a special magnet acquired from the National High Magnetic Field Laboratory in Florida, the small size of its center hole limits the amount of magnetic flux Fonck and his students can push through it, he says. Despite a powerful magnetic field that exceeds those employed in larger national laboratory devices, the group found the plasma became mildly unstable, or wobbled. “And we couldn’t get ride of the wobbling because we didn’t have enough oomph to drive the current,” he says.

The issue isn’t unique to Pegasus. In fact, plasma fusion researchers worldwide have been wrestling with how to start tokamaks — and especially those with very low aspect ratios — without the solenoid. “That’s been a critique of these devices for the past decade or more,” says Fonck.

Researchers at the University of Washington and Princeton conducted seminal experiments in the area, but encountered limitations. For its studies, Fonck’s group drew on small plasma torches developed for the Madison Symmetric Torus experiment in the UW-Madison Department of Physics. Essentially, says Fonck, the torches are a plasma source from which he and his students can pull a lot of current. For its method, Fonck’s group turns on the magnetic field that encircles the device toroidally. Next, the researchers turn on the vertical magnetic field that holds the plasma in place — somewhat like a tire confines an inner tube. “And so you end up with a magnetic field that spirals, because it’s got a component that goes around the torus and one that goes up,” he says. “The magnetic field lines are like a helix — they just spiral up from the bottom of the machine to the top.”

Using the plasma torches, the researchers inject current from below along those helical magnetic field lines. The current spirals up and hits the armored top of the machine — which, says Fonck, is OK. Then, the current becomes unstable and naturally collapses into a lower-energy state. “It turns out that the lowest-energy state under those conditions is a standard tokamak plasma,” says Fonck. “So, the plasma organizes itself into a tokamak — even though all you’re doing is pumping current along this magnetic field line. This is an example of plasmas self-organizing into more complex systems.”

Current from the plasma torches becomes a 100 kiloamp plasma current of a fully formed circular tokamak plasma. It stays that way, says Fonck, until his group turns off the current.

The technique has become one of the group’s main focus areas. Locally, it provides a path for the researchers to deliver current to Pegasus and someday achieve the high-pressure plasmas they’re aiming for. Globally, says Fonck, the technique may scale up to full-size reactors. “That’s a big deal in the international spherical tokamak community,” he says.

UW-Madison engineering physics PhD student Devon Battaglia is lead author on the Physical Review Letters paper. In addition to Fonck, the team also included graduate students Mike Bongard, and scientists Aaron Redd and Aaron Sontag (now at Oak Ridge National Laboratory). Funding for Pegasus comes from the U.S. Department of Energy.